1. Field of the Invention
This invention relates to integrated circuit processing and, more particularly, to a method of using dielectric anti-reflective coating (DARC) in conjunction with bottom anti-reflective coating (BARC) to form an anti-reflective barrier layer that conforms to the topography of the substrate surface and is tuned to function effectively in both annealed and unannealed states.
2. Description of the Related Art
The typical semiconductor process involves implanting or depositing regions or layers of different material either into or on different regions of a semiconductor substrate. To ensure that the material is positioned at the correct location on the semiconductor substrate, a photo imaging process is typically used to define the regions that are to subsequently receive the material. The conventional photo imaging process, known as photolithography, generally involves projecting light waves onto a photoresist surface so that the light can react with the photoresist to create an imaged pattern. The photoresist can then be selectively removed as a result of the exposure such that a region of the semiconductor device is exposed to receive the additional material.
In some cases the light waves are known to propagate through the photoresist, reach the underlying substrate, and reflect from the substrate surface back through the photoresist. The reflected light can interfere with other waves propagating through the photoresist and ultimately reduce the accuracy and precision of the image transferred. In particular, the reflected light can interfere and scatter light waves that are being directed towards a particular region of the photoresist which in turn reduces the effectiveness of exposure intended for the region. As a consequence, the region of the photoresist may not be as uniformly exposed and selective removal of the photoresist during subsequent processing steps may be affected. Furthermore, light reflected from the substrate surface can scatter, especially if the substrate surface is non-planar, such that the scattered light can inadvertently expose the photoresist surrounding the desired region of the photoresist. Hence, the reflected light can expose regions of the photoresist that should otherwise remain unexposed which limits the ability to precisely define regions of the photoresist for selective removal.
To address this particular problem associated with the photo imaging process, anti-reflective coatings are commonly used to attenuate or absorb the light waves reflected from the substrate surface during photo exposure operations. Anti-reflective coatings are materials generally known for their ability to absorb various wavelengths of radiation. They are typically interposed between the substrate surface and the photoresist so as to serve as a barrier that inhibits the reflected waves from traversing back through the photoresist and adversely affecting the imaging process. Dielectric anti-reflective coating (DARC) and bottom anti-reflective coating (BARC) are examples of anti-reflective materials that are commonly used to absorb radiation reflected from the substrate surface during the photo imaging operations of integrated circuit processing.
In particular, BARC is generally available as a low-viscosity liquid that can be applied onto the substrate surface using a well known spin coating process. Disadvantageously, however, BARC cannot be universally applied to all substrate surfaces. The relatively low viscosity of BARC and inherent limitations of the spin coating process render BARC inadequate in covering substrate surfaces with a substantially contoured topography. In particular, BARC is known to accumulate or pool in recessed regions on a non-planar surface while leaving most high profiled areas on the substrate exposed. Furthermore, BARC is known to vaporize under high temperature conditions that are characteristic of most annealing operations and therefore is typically rendered useless following high temperature annealing.
To address this problem associated with BARC, dielectric anti-reflective coating (DARC) is often used in place of BARC for substrates having a contoured topography or for substrates that are subjected to high temperature annealing. DARC is chosen largely for its ability to conform to the contours of the substrate surface and its ability to withstand the typical annealing operation. Furthermore, it is generally known that the composition of DARC can be adjusted or tuned to function effectively in either annealed or unannealed states
Disadvantageously, however, DARC that is adjusted to function effectively in an annealed state tends to be ineffective before being unannealed. Likewise, DARC that is tuned to work effectively before annealing usually loses its anti-reflective properties after high temperature annealing. Consequently, more than one layer of DARC is generally applied to substrates that undergo high temperature annealing between successive photo imaging steps. However, the formation of multiple layers of DARC onto the substrate surface typically involves repeatedly exposing the substrate to high temperature conditions that are characteristic of most DARC deposition methods. It can be appreciated that subjecting substrates to multiple thermal sequences is not only costly and time consuming but also can increase the occurrence of defects in the substrate.
Hence, from the foregoing, it will be appreciated that there is a need for a method of inhibiting light waves from reflecting back through the photoresist during photo exposure operations such that the method can be applied universally to most substrate surfaces and is not affected by thermal cycling. To this end, there is a particular need for an anti-reflective barrier layer that substantially conforms to the topography of the substrate surface and is adapted to function effectively in both annealed and unannealed states.
The aforementioned needs are satisfied by the method of the present invention which uses dielectric anti-reflective coating (DARC) in conjunction with bottom anti-reflective coating (BARC) to form an anti-reflective barrier layer that substantially conforms to the topography of the substrate surface and is tuned to function effectively in both annealed and unannealed states.
In one aspect, this invention comprises a method of forming an anti-reflective barrier layer on a substrate surface wherein a layer of DARC is first formed on an upper surface of the substrate and a layer of BARC is subsequently formed on an upper surface of the DARC layer. Preferably, the DARC layer substantially conforms to the topography of the substrate surface and provides a surface that is adapted to receive the BARC layer. Preferably, the composition of DARC is adjusted in a manner such that its anti-reflective properties are optimized after undergoing high temperature annealing while the BARC is selected for its ability to absorb light waves before annealing. In one embodiment, the composition of DARC comprises silicon, oxygen, nitrogen and the composition of BARC comprises organic polymers made of carbon, oxygen, and nitrogen. However, it can be appreciated that a variety of other formulations of DARC and BARC can be used without departing from the scope of the present invention.
In another aspect of this invention, the method of using DARC in conjunction with BARC as a barrier layer is adapted to attenuate reflected radiation generated during the photo imaging operations of FLASH memory circuit processing. In particular, the FLASH memory circuit processing typically involves the formation of a gate stack which typically comprises high temperature annealing operations interposed between multiple print and etch steps. As such, it is desirable to form a barrier layer that functions as an effective anti-reflective coating during photo imaging operations both before and after annealing.
In one embodiment, a layer of DARC is formed on an upper surface of a nitride cap layer of a conventional gate stack for FLASH memory circuits. Preferably, the DARC is deposited onto the nitride cap using a well known chemical vapor deposition process and substantially conforms to the topography of the nitride cap as well as all other features on the substrate surface. Furthermore, a layer of BARC is subsequently formed on an upper surface of the DARC layer using a well known spin coating process. Preferably, the BARC layer serves as an effective anti-reflective coating under pre-annealing conditions while the DARC layer is tuned for post-annealing uses. Furthermore, the DARC layer is formed adjacent to the substrate surface as it is able to conform to the surface topography of the gate stack. Advantageously, the DARC and BARC layers are selected and used in a complementary fashion so that they compensate for each other""s deficiencies and form a barrier layer that retains the advantages of both materials.
Furthermore, the anti-reflective barrier layer comprising DARC and BARC is preferably interposed between the nitride cap of the gate stack and a layer of subsequently deposited photoresist used for a first photo imaging and patterning operation. In particular, the BARC layer is adapted to inhibit the reflected light waves generated from the first photo imaging operation from traversing back through the photoresist and affect the image transferred. Furthermore, subsequent to the first print and etch operation, the BARC is removed and the gate stack undergoes a high temperature annealing process. Preferably, the remaining DARC layer that is tuned to function in an annealed state will be used to attenuate reflected light waves that are generated from photo imaging operations following the annealing process.
In yet another aspect of the invention, the method of using DARC in conjunction with BARC provides an effective barrier that inhibits the nitride layer of the gate stack from seeping into the adjacent photoresist layer and poisoning the photoresist. Poisoning is known to occur when base chemicals from the nitride seep into the photoresist and neutralize the acids in the photoresist. The depletion of acids in the photoresist is known to cause incomplete development of photoresist and leave behind what is commonly known as a photoresist foot. In the preferred embodiment, a barrier layer comprising of DARC and BARC is interposed between the nitride cap and the photoresist so as to inhibit bases from the underlying nitride layer to seep into the adjacent photoresist layer and neutralize the acids in the photoresist. Advantageously, the barrier layer effectively inhibits the undesirable formation of photoresist foot.
In another aspect of the invention, a method of patterning and etching a semiconductor circuit is provided. The method comprises forming an anti-reflection barrier on the semiconductor circuit that has a first component tuned to absorb reflected light from the semiconductor circuit in an unannealed state and a second component tuned to absorb reflected light from the semiconductor surface in an annealed state. The method also comprises positioning a first photoresist layer on the anti-reflective barrier and then patterning and etching the first photoresist layer. The first component in combination with the second component of the anti-reflective barrier inhibit light from being reflected from the semiconductor device into the first photoresist layer during patterning and etching of the first photoresist layer. The method also comprises annealing the semiconductor circuit following the patterning and etching of the first photoresist layer wherein the annealing step improves the ability of the second component of the anti-reflective barrier to inhibit reflected light from travelling therethrough. The method further comprises depositing a second photoresist layer on the anti-reflective barrier and patterning and etching the second photoresist layer wherein the second component of the anti-reflective barrier inhibits light from being reflected from the semiconductor device into the second photoresist layer during patterning and etching of the second photoresist layer.
In another aspect the invention comprises a method of inhibiting light from reflecting off of an exposed surface of a semiconductor device during patterning and etching. In this aspect, the method comprises positioning a first barrier layer on the exposed surface of the semiconductor device wherein the first barrier layer is configured to inhibit light from reflecting from the exposed surface of the semiconductor layer. The method further comprises positioning a second barrier layer on the first barrier layer wherein the second barrier layer is configured to inhibit light from reflecting from the exposed surface of the semiconductor device. The method then comprises positioning a first photoresist layer on the second barrier layer and patterning and etching the first photoresist layer. The second barrier layer decreases the amount of reflected light reflecting from the exposed surface from entering the first photoresist layer during patterning and etching of the first photoresist layer. The method further comprises transforming the first barrier layer by annealing the semiconductor device so as to improve the ability of the first barrier layer to prevent light from being reflected from the exposed surface of the semiconductor device into a subsequently deposited photoresist layer during patterning and etching of the subsequently formed layers.
From the foregoing, it will be appreciated that the aspects of the present invention provide a method of forming an anti-reflective barrier layer comprising DARC and BARC onto a substrate surface of an integrated circuit assembly so that the barrier layer is able to withstand high temperature annealing, conforms to the substrate surface and is more effective in absorbing reflected light waves generated from the photo imaging operations both before and after annealing. Furthermore, the present invention provides, in one aspect, an effective barrier layer in inhibiting base chemicals from the nitride layer of a gate stack from seeping into adjacent photoresist layers and undesirably neutralize the acids in the photoresist. These and other objects and advantages of the present invention will become more apparent from the following description taken in conjunction with the following drawings.